The degree to which fitness and physical activity in older populations is genetically determined and extent to which these are related to cardiovascular risk factors is unknown. This information is especially important in an older population where fitness and activity levels decline and the incidence of cardiovascular disease increases. To begin to understand these relationships, male twin pairs greater than 64 years of age living in Allegheny County, PA will be identified from a list of names and birthdates for all people over age 64. Those with overt orthopedic or cardiovascular disease problems will be eliminated following initial contact. A random sample of 40 pairs of both monozygotic and dizygotic twins will be selected from the remaining group and undergo zygosity testing. Physical fitness parameters (aerobic capacity via treadmill test and grip strength), habitual physical activity as assessed by activity questionnaire and objective monitoring and cardiovascular risk factors (blood pressure, lipoprotein cholesterol including HDL2 and HDL3, smoking and drinking behavior will be assessed. The goal is to evaluate (1) the genetic influence on aerobic capacity, strength and physical activity patterns, (2) to compare the degree of genetic association between fitness, physical activity and HDL cholesterol may be related to aerobic capacity separate from genetic influence. This information will provide background for future investigation where fitness and activity levels of older populations of MX and DZ twins could be followed prospectively to determine the degree to which changes in fitness with aging are genetically or environmentally controlled and the relationship of these changes to cardiovascular disease risk factors.